Gel sheet

ABSTRACT

The present invention is intended to provide a gel sheet having a formulation providing an adhesive force on the skin side suitable for the skin (i.e., a gel having a low adhesive force and being able to withstand a tip pressure of a terminal and the like), and concurrently having a sufficient adhesive force against an electrode element on the side opposite to the skin side. The present invention is a gel sheet having a laminated structure of a layer A and a layer B, characterized in that the storage elastic modulus of the layer A at 23° C. and 10 Hz is 12,000 to 40,000 Pa, and the storage elastic modulus of the layer B at 23° C. and 10 Hz is 2,000 to 10,000 Pa.

TECHNICAL FIELD

The present invention relates to a gel sheet. In particular, the presentinvention relates to an adhesive gel sheet for skin for bioelectrode,medical tape and the like used for electroencephalography orrehabilitation of the elderly for which it is necessary to reduce damageto the skin.

BACKGROUND ART

The gel sheet is preferably used as a surgical tape to be attached to aliving body, a fixing tape for various medical devices, a bioelectrodepad to be attached to a living body, an electrode for anelectrocardiogram, an industrial adhesive tape for a building materialand an electronic material and the like. Properties required for a gelsheet are different depending on the respective uses.

For example, a fixing tape is required to be firmly fixed to the skin,and to reduce sweating and rash when attached to the skin or damage whenpeeling off the tape. Further, since a bioelectrode pad is attached tothe skin for a long time, it is necessary to keep an adhesive force onthe electrode element side, in addition to adhesion to the skin.

Along with the recent aging of the population, there is a demand formedical electrodes (for electroencephalography or for stimulating themuscles for rehabilitation) that are simpler and more accurate inmeasurements. For example, in electroencephalography, cellulose fiberscontaining physiological saline were conventionally used as anelectrode. However, since cellulose fibers have poor water retention,there were problems of change in moisture content over time and inhandling at the time of measurements.

Therefore, in Patent Literature 1 and the like, it is proposed to use ahydrogel for taking an electrocardiogram. However, since a gel materialis inherently designed to attach to the skin, it is soft and adhesive.Therefore, when setting an electrode on the scalp, there was a problemin that the hair may get entangled with the adhesive medium. Further,since the gel material follows the fine irregularities of the skin,there was a problem in that damage was incurred to the skin when peelingoff the gel. Moreover, since the hydrogel is a very soft material, thereis a possibility that the hydrogel may break by the pushing pressure ofthe convex portion of the electrode in making measurements by pointcontact such as electroencephalography.

CITATION LIST Patent Literature

-   Patent Literature 1: JP Patent Publication (Kohyo) No. 2016-501049 A

SUMMARY OF INVENTION Technical Problem

The present invention, in view of the conventional circumstancesmentioned above, is intended to provide a gel sheet having a formulationproviding an adhesive force on the skin side suitable for the skin(i.e., a gel having a low adhesive force and being able to withstand atip pressure of a terminal and the like), and concurrently having asufficient adhesive force against an electrode element on the sideopposite to the skin side.

Solution to Problem

As a result of intensive investigation, the inventors of the presentinvention have found that a gel sheet having adhesion and repulsionincurring minor damage to a delicate skin can be obtained by laminatinggels having different compositions, imparting different theologicalproperties to a layer attached to the skin and a layer on the oppositeside, and making the layer attached to the skin harder than the layer onthe opposite side, and completed the invention. In other words, thesummary of the present invention is as follows.

-   (1) A gel sheet having a laminated structure of a layer A and a    layer B, wherein a storage elastic modulus of the layer A at 23° C.    and 10 Hz is 12,000 to 40,000 Pa, and a storage elastic modulus of    the layer B at 23° C. and 10 Hz is 2,000 to 10,000 Pa.-   (2) The gel sheet according to (1), wherein an adhesive force of the    layer A to a Bakelite plate is 0.1 to 2.0 N/20 mm, and an adhesive    force of the layer B to the Bakelite plate is 2.5 to 15 N/20 mm,-   (3) The gel sheet according to (1) or (2), having a thickness of 0.2    mm to 2.0 mm.-   (4) The gel sheet according to any one of (1) to (3), wherein a    ratio of a thickness of the layer A to a thickness of the layer B    (A/B) is 0.25 to 3.5.-   (5) The gel sheet according to any one of (1) to (4), wherein the    layer A and the layer B are hydrogels comprising a polymer matrix,    water and polyhydric alcohol, and the polymer matrix is a copolymer    of one or more monofunctional monomers selected from a    (meth)acrylamide-based monomer and (meth)acrylate ester, and a    crosslinkable monomer.-   (6) The gel sheet according to (5), wherein the layer A further    comprises polyvinyl alcohol.-   (7) The gel sheet according to any one of (1) to (6), wherein an    alternating current impedance of the layer A and that of the layer B    at 10 Hz are both 1,000 Ω or less.-   (8) The gel sheet according to any one of (1) to (7), being an    adhesive gel sheet for skin to be used by attaching a surface of the    layer A to the skin.

The present specification contains the disclosure of JP PatentApplication Nos. 2016-193461 and 2017-041815, which are the basis ofpriority of the present application.

Advantageous Effects of Invention

According to the present invention, by constituting a gel sheet by morethan one layer and making a storage elastic modulus of a layer attachedto the skin higher to form a hard layer and a soft layer, it is possibleto exert hardness (repulsion and durability) and adhesive forcedepending on the respective adherends of the skin and an electrodeelement, and to preferably use the gel sheet in applications such aspressing and fixing a convex portion of an electrode to a scalp via thegel sheet without having hair entangled, as an electrode used forelectroencephalography and the like.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a view schematically showing a cross-sectional view of anembodiment of a gel sheet according to the present invention.

DESCRIPTION OF EMBODIMENT

The present invention is described in detail hereinafter.

FIG. 1 shows a cross-sectional view of one embodiment of a gel sheetaccording to the present invention. A gel sheet 1 according to theembodiment is constituted by laminating a layer A 10 and a layer B 20.Further, the storage elastic modulus of the layer A 10 at 23° C. and 10Hz is 12,000 to 40,000 Pa, and the storage elastic modulus of the layerB 20 at 23° C. and 10 Hz is 2,000 to 10,000 Pa.

The gel sheet 1 is preferably used as a bioelectrode, the layer A 10 isattached to a skin, and the surface of the layer B 20 is brought intocontact with an electrode element. Since the layer A 10 is harder thanthe layer B 20 and the layer B is softer, the layer B 20 adheres well tothe electrode element and no air bubbles are developed between the layerB and the electrode element.

Moreover, it is preferable that the adhesive force of the layer A 10 toa Bakelite plate be 0.1 to 2.0 N/20 mm, and the adhesive force of thelayer B 20 to the Bakelite plate be 2.5 to 15 N/20 mm. For the layer B20, it is particularly preferably 2.5 to 8.0 N/20 mm. Since the adhesiveforce of the layer A is small, damage is not incurred when peeling offthe gel sheet from the skin and hair is not tangled. When adopting aconstitution that the layer B side is not removed from the electrodeelement, it is desirable that the adhesive force of the layer B be high.

Further, in the present invention, the adhesive force to the Bakeliteplate means the average value of measured stress values of 3 tests(total of 15 points) performed in accordance with JIS Z0237: 2009 asfollows: a gel sheet is cut into 120 mm×20 mm, and a Bakelite plate isattached on the side of either the layer A or the layer B to bemeasured; test pieces are prepared by rolling a pressure roller of 2 kgback and forth once; and stress values (N/20 mm) are measured using arheometer (CR-500DX, manufactured by Sun Scientific Co., Ltd.), underthe measuring conditions of an angle of 90° and a speed of 300 mm/min atpoints from the starting point of measurement to predetermined peelingpoints (30, 40, 50, 60, and 70 mm). The measurements shall be performedin an environment of a temperature of 23±5° C. and a humidity of55%±10%.

It is necessary that the layer A 10 and the layer B 20 have adhesion andshock-absorbing properties (repulsive force), and in the case of usingthe gel sheet as a bioelectrode, it is preferable to further haveelectric conductivity. In particular, it is preferable that the layer A10 and the layer B 20 be constituted of a hydrogel containing water inview of its application to human body. The hydrogel is excellent inwater retention and the like, and is used in a variety of fields such asmedical care, pharmaceuticals, food products, civil engineering,bioengineering, sports, and the like.

As the hydrogel, for example, a hydrogel containing a polymer matrix,water, and polyhydric alcohol is preferably used.

As an example, the polymer matrix can be formed of a copolymer of amonofunctional monomer having one ethylenically unsaturated group and acrosslinkable monomer, but it is not limited thereto.

As the monofunctional monomer, a water-soluble monomer such as(meth)acrylamide-based monomer or (meth)acrylate ester is preferablyused.

Specific examples of the (meth)acrylamide-based monomer includeN,N-dialkyl(meth)acrylamide such as (meth)acrylamide,N,N-dimethyl(meth)acrylamide and N,N-diethyl(meth)acrylamide;N-alkyl(meth)acrylamide such as N-isopropyl(meth)acrylamide,N-methyl(meth)acrylamide, N-ethyl(meth)acrylamide andN-propyl(meth)acrylamide; N-hydroxyalkyl(meth)acrylamide such asN-hydroxyethyl(meth)acrylamide and N-hydroxymethyl(meth)acrylamide;N-alkoxyalkyl(meth)acrylamide such as N-ethoxymethyl(meth)acrylamide,N-propoxymethyl(meth)acrylamide, N-butoxymethyl(meth)acrylamide,N-isobutoxymethyl(meth)acrylamide, N-pentoxymethyl(meth)acrylamide,N-hexyloxymethyl(meth)acrylamide, N-heptoxymethyl(meth)acrylamide,N-octoxymethyl(meth)acrylamide, N-ethoxyethyl(meth)acrylamide,N-propoxyethyl(meth)acrylamide, and N-butoxyethyl(meth)acrylamide; acationic acrylamide-based compound containing an amino group such asdimethylaminopropyl(meth)acrylamide; an anionic monofunctional monomercontaining a sulfonic acid group such as 4-acryloyl morpholine, andtert-butyl acrylamide sulfonic acid, or a salt thereof; derivativesthereof; and the like. Among the above, at least one selected from thegroup consisting of (meth)acrylamide, N,N-dimethyl(meth)acrylamide,N,N-diethyl(meth)acrylamide, N-isopropyl(meth)acrylamide,N-methyl(meth)acrylamide, N-ethyl(meth)acrylamide,N-propyl(meth)acrylamide, N-hydroxyethyl(meth)acrylamide,N-hydroxymethyl(meth)acrylamide, dimethylaminopropyl(meth)acrylamide,4-acryloyl morpholine, tert-butyl acrylamide sulfonic acid, and saltsthereof is preferably used, but is not limited thereto.

Specific examples of the (meth)acrylate ester include, (meth)acrylatealkyl ester with the alkyl group containing 1 to 18 carbon atoms, forexample, (meth)acrylate alkyl ester such as methyl (meth)acrylate, ethyl(meth)acrylate, n-propyl (meth)acrylate, isopropyl (meth)acrylate,n-butyl (meth)acrylate, isobutyl (meth)acrylate, tert-butyl(meth)acrylate, n-hexyl (meth)acrylate, n-octyl (meth)acrylate, isooctyl(meth)acrylate, 2-ethylhexyl (meth)acrylate, n-nonyl (meth)acrylate,isononyl (meth)acrylate, n-pentyl (meth)acrylate, n-decyl(meth)acrylate, isodecyl (meth)acrylate, n-lauryl (meth)acrylate,tridecyl (meth)acrylate, and n-stearyl (meth)acrylate; alicyclic(meth)acrylate such as cyclohexyl (meth)acrylate, isobornyl(meth)acrylate, and 1-adamantyl (meth)acrylate; alkoxy group-containing(meth)acrylate ester such as methoxy polyethylene glycol(meth)acrylatesuch as 2-methoxyethyl (meth)acrylate, ethoxyethoxyethyl(meth)acrylate,and methoxytriethylene glycol(meth)acrylate; hydroxyalkyl (meth)acrylate(where an aryl group may be bonded to a hydroxyalkyl group via an etherbond) such as 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl(meth)acrylate, 2-hydroxy-3-phenoxypropyl (meth)acrylate, and2-hydroxybutyl (meth)acrylate; glycerin mono(meth)acrylate; polyalkyleneglycol mono(meth)acrylate such as polyethylene glycol mono(meth)acrylateand polyethylene glycol-polypropylene glycol copolymer; (meth)acrylateester having an aromatic ring such as benzyl (meth)acrylate;(meth)acrylate ester having a heterocyclic ring such astetrahydrofurfuryl (meth)acrylate; and the like.

In addition to the (meth)acrylamide-based monomers mentioned above, asrequired, the monofunctional monomer mentioned above may be(meth)acrylate or a salt thereof, (meth)acrylate ester, a vinylamide-based monofunctional monomer such as vinyl pyrrolidone, vinylacetamide, vinyl formamide; a nonionic monofunctional monomer such asallyl alcohol, and a styrene-based monomer, and the like. Thesemonofunctional monomers may be used singly or in combinations of two ormore. Further, in the present specification, (meth)acryl means acryl ormethacryl, and (meth)acrylate means acrylate or methacrylate.

The content of the structural unit derived from the monofunctionalmonomer mentioned above in each of the layer A and the layer B is notparticularly limited, but it is preferably in the range of 10 to 30% byweight and more preferably in the range of 15 to 25% by weight withrespect to the total amount of the hydrogel. When the content of thestructural unit derived from the monofunctional monomer mentioned aboveis too small, the shape retaining property of the hydrogel becomesinsufficient and there is a possibility that it is too soft or easilytorn apart. Moreover, when the content of the structural unit derivedfrom the monofunctional monomer mentioned above is too large, it willnot be dissolved from the perspective of solubility in water, andtherefore it is appropriately determined in view of balancing theseaspects. As required, the content of the monofunctional monomer in thelayer A may be increased, and the gel may be hardened so that thestorage elastic modulus of the layer A becomes 12,000 to 40,000 Pa.

As the crosslinkable monomer mentioned above, it is preferable to use amonomer having two or more polymerizable double bonds in the molecule.Specific examples thereof include polyfunctional (meth)acrylamide or(meth)acrylate such as N,N′-methylenebis(meth)acrylamide,N,N′-ethylenebis(meth)acrylamide, (poly)ethylene glycoldi(meth)acrylate, (poly)propylene glycol di(meth)acrylate, glycerindi(meth)acrylate and glycerin tri(meth)acrylate, tetraallyloxyethane;diallyl ammonium chloride; and the like, and these may be used singly orin combinations of two or more. Further, as the crosslinkable monomerhaving two or more polymerizable double bonds in the molecule mentionedabove, a polyglycerol derivative which is a polyfunctional compoundhaving two or more (meth)acryloyl groups or vinyl groups and having amolecular weight of 400 or more as described in JP Patent No. 2803886can also be used.

The amount of the crosslinkable monomer to be added is preferably in therange of 0.01% by weight to 0.1% by weight with respect to the totalamount of the hydrogel. When the amount to be added is too small, thecrosslinking density becomes low, the shape stability becomes poor, andat the same time the cohesive force decreases, the holding force of thegel material itself decreases, and handling of the gel becomesdifficult. Moreover, since a portion of the gel material remains on anadherend when peeling off, handling property of the gel sheetdeteriorates. Moreover, when the amount of the crosslinkable monomer tobe added is too large, the adhesive force becomes small and there is apossibility that the gel becomes hard and brittle.

Moreover, the content of water in the hydrogel is not particularlylimited, but it is preferably 10 to 60% by weight, and more preferably15 to 30% by weight, with respect to the total amount of the hydrogel.When the content of water is too small, the water content of thehydrogel with respect to the equilibrium water content decreases, thehygroscopicity of the hydrogel becomes large, and the hydrogel maydegenerate (for example, swell) over time. Moreover, When the content ofwater is too large, the water content of the hydrogel with respect tothe equilibrium water content increases, contraction and propertychanges of the hydrogel may develop from drying.

Examples of the polyhydric alcohol are not particularly limited, and theexamples include diols such as ethylene glycol, triethylene glycol,1,6-hexanediol, 1,9-nonanediol, propylene glycol, and butanediol; a3-valent or higher polyhydric alcohol such as glycerin, pentaerythritol,sorbitol; a polyhydric alcohol condensate such as polyethylene glycol,polypropylene glycol and polyglycerin; a modified polyhydric alcoholsuch as polyoxyethylene glycerin; and the like.

Among the polyhydric alcohols, it is preferable to use a polyhydricalcohol that is in a liquid state in the working temperature range ofthe hydrogel (for example, around 20° C. when used indoors),specifically, ethylene glycol, triethylene glycol, propylene glycol,polypropylene glycol, polyethylene glycol, polyglycerin, glycerin andthe like are preferable.

The content of the polyhydric alcohol in the hydrogel is notparticularly limited, but it is preferably in the range of 20 to 70% byweight, and more preferably in the range of 25 to 65% by weight, withrespect to the total amount of the hydrogel. When the content of thepolyhydric alcohol is too small, moisture retention and plasticity ofthe obtained hydrogel are poor, transpiration of moisture becomesremarkable, and stability of the hydrogel over time as well asflexibility lack, and therefore sufficient adhesion may not be obtained.When the content of the polyhydric alcohol is too large, the amount ofthe polyhydric alcohol exceeds the amount of polyhydric alcohol that thepolymer matrix can retain, and the physical properties of the hydrogelchange due to bleed-out of the polyhydric alcohol from the surface ofthe hydrogel, and sufficient adhesion may not be obtained. Therefore,the content of the polyhydric alcohol is appropriately determined inview of balancing these aspects.

In particular, with respect to the layer A 10, in order to have a bufferaction while maintaining the flexibility of the gel, it is possible toprovide a S-IPN (Semi-Interpenetrating Polymer Network) structure to theskeleton by mixing a water-soluble synthetic polymer such as polyvinylalcohol at the time of polymerization. By containing a water-solublesynthetic polymer such as polyvinyl alcohol, the storage elastic modulusof the layer A is increased and the gel can be hardened.

When polyvinyl alcohol is contained in the layer A, and if the degree ofpolymerization of the polyvinyl alcohol is too small, the effect ofimproving the mechanical strength cannot be obtained. Conversely, if thedegree of polymerization is too large, the viscosity increase issignificant and a homogeneous mixture cannot be prepared when dissolvingthe polyvinyl alcohol in a monomer containing solution prepared at thetime of making the gel. Therefore, the degree of polymerization isappropriately determined in view of balancing these aspects. Preferably,the viscosity average degree of polymerization is 500 to 3,000.

Moreover, when the degree of saponification of the polyvinyl alcoholused in the present embodiment is too small, the solubility at the timeof preparation of the compounded solution improves, but the stability ofthe hydrogel obtained deteriorates, and it is not preferable.Conversely, when the degree of saponification is too large, thesolubility of the polyvinyl alcohol extremely decreases and it becomesdifficult to prepare the compounded solution, and therefore the degreeof saponification is preferably in the range of 80 to 98%, and morepreferably 88 to 98%, but it is not limited to these ranges.

When the polyvinyl alcohol is contained in the layer A, the content ofthe polyvinyl alcohol with respect to the total amount of the hydrogelconstituting the layer A is not particularly limited as long as thestorage elastic modulus of the layer A can be within the range of 12,000to 40,000 Pa. When the content is too small, improvement in themechanical strength of the layer A cannot be expected. Conversely, whenthe content is too large, the polyvinyl alcohol cannot be completelydissolved in the gel, and precipitation and generation of undissolvedpolyvinyl alcohol develop, and a uniform gel body cannot be obtained.Therefore, the content is appropriately determined in view of balancingthese aspects. Specifically, it is preferably 0.15 to 5% by weight withrespect to the total amount of the hydrogel.

Moreover, the layer A 10 and the layer B 20 can contain an electrolytedepending on the needs, thereby it is possible to impart electricconductivity to the gel material constituting the layer A 10 and thelayer B 20. When the gel sheet 1 is used as a bioelectrode, it ispreferable that the alternating current impedance of each of the layer A10 and the layer B 20 at 10 Hz be 1,000 Ω or less.

When imparting electric conductivity to the gel material, the content ofthe electrolyte in the gel material is preferably 0.05 to 10% by weight,and more preferably 2 to 6% by weight, with respect to the total amountof the layer A or the layer B. When the content of the electrolyte istoo small, the impedance becomes high and the electric conductivitycannot be said to be good. Moreover, although the impedance decreases asthe content of the electrolyte increases, when the content of theelectrolyte is too large, the amount of water required to dissolve theelectrolyte increases, which is not preferable.

The electrolyte is not particularly limited, and examples thereofinclude alkali metal halides such as sodium halide (for example, sodiumchloride), lithium halide and potassium halide; alkali earth metalhalides such as magnesium halide and calcium halide; and other metalhalides. Moreover, as the electrolyte, hypochlorite, chlorite, chlorate,perchlorate, sulfate, carbonate, nitrate and phosphate of various metalsare also preferably used. Moreover, as the electrolyte, inorganic saltssuch as an ammonium salt and various complex salts; salts of monovalentorganic carboxylic acids such as acetic acid, benzoic acid and lacticacid; salts of polyvalent organic carboxylic acids such as tartaricacid; monovalent or divalent or higher valent salts of polyvalentcarboxylic acids such as phthalic acid, succinic acid, adipic acid,citric acid; metal salts of organic acids such as sulfonic acid andamino acid; organic ammonium salts and the like are also preferable.

Moreover, to the layer A 10 and the layer B 20, a base such as sodiumhydroxide may be appropriately added for the purpose of adjusting thepH.

Moreover, in order to make the layer B 20 follow up the electrodeelement, the layer B 20 may be made to be fixed more firmly.Specifically, a hydrophilic polymer or a water-insoluble polymer havingadhesion can be contained as a tackifier. For example, as thehydrophilic polymer, polyacrylic acid or a salt thereof can be used. Theamount of the polyacrylic acid or the salt thereof to be added is notparticularly limited, but is preferably in the range of 0.25 to 2% byweight with respect to the total amount of the layer B. When the amountto be added is small, there is no effect of imparting adhesion, and whenthe amount to be added is large, adhesion does not become large inproportion to the amount to be added, and therefore it is desirable toset the amount to be added in view of balancing the whole.

Moreover, as the water-insoluble polymer having adhesion, hydrophobicmonomers such as (meth)acrylate ester, vinyl acetate, maleate ester,rosin-based resin and the like may be added.

In particular, examples of the water-insoluble polymer having adhesioninclude any one of hydrophobic monomers such as (meth)acrylate ester,vinyl acetate, maleate ester and the like alone or a polymer of morethan one thereof. Specifically, examples include a homopolymer or aplurality of copolymers of any of the hydrophobic monomers such asisooctyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, butyl(meth)acrylate, vinyl acetate, dioctyl maleate. In addition to themonomers mentioned above, one or more of any of the hydrophobic monomerssuch as ethylene, propylene, butylene, methyl (meth)acrylate, ethyl(meth)acrylate may be further copolymerized. Further, silicone adhesivesand natural rubber-based and synthetic rubber-based adhesives can alsobe contained. Among these, an acrylate ester copolymer is excellent invarious properties and has high adhesion, and therefore is preferablyused.

In order to disperse the water-insoluble polymer having adhesion in thelayer B, it is preferable to use an emulsion where the polymer isemulsified and dispersed. Usually, the content of solids in the emulsionis 30 to 60% by weight and most of the balance is water.

For example, as an emulsion of an acrylate ester-based copolymer resin,trade name “Polysol PSA SE-1730” manufactured by Showa Denko KK, tradename “Vinyblan ADH-1048” manufactured by Nissin Chemical Industry Co.,Ltd. and the like are preferably used.

The content of the water-insoluble polymer having adhesion in the layerB 20 may be adjusted according to the performance expected of the finalproduct, but in order to obtain good adhesion to the electrode element,it is necessary to add 3% by weight or more with respect to the totalamount of the layer B. The content is preferably 5% by weight or more,more preferably 8% by weight or more, particularly preferably 10% byweight or more. Moreover, since the water-insoluble polymerindependently has a function as an adhesive, the content may be large.However, when the content is too large, the adhesive force does notimprove by a certain value or more. Moreover, when the water-insolublepolymer is used as a hydrogel for a medical electrode, the electricconductivity of the gel also decreases, and therefore it is preferableto set the content to 20% by weight or less with respect to the totalamount of the layer B in view of balancing these aspects. The content ismore preferably 15% by weight or less, and particularly preferably 13%by weight or less.

Further, it is possible to add polyvinyl alcohol to the layer B 20 as adispersion stabilizer depending on one's needs. Thereby, the adhesiveforce of the layer B can be further improved. Since the polyvinylalcohol added to the layer B is for ensuring dispersion stability, thedegree of saponification is preferably within the range of 50 to 75%.

The content of the polyvinyl alcohol in the layer B 20 is appropriatelydetermined from the perspective of ensuring dispersion stability. Thecontent is preferably 0.05 to 5% by weight.

Further, the layer A 10 and the layer B 20 may also contain otheradditives depending on one's needs. Examples of other additives include,for example, rust inhibitors, antifungal agents, antioxidants, defoamingagents, stabilizers, surfactants, colorants and the like.

The layer A 10 and the layer B 20 can be obtained by dissolving orhomogeneously dispersing each of the materials mentioned above, apolymerization initiator, a solvent and the like, and polymerizing andcrosslinking by heating, ultraviolet irradiation or the like. Thepolymerization initiator may be a thermal polymerization initiator or aphotopolymerization initiator. The amount of the polymerizationinitiator to be added is preferably 0.01% by weight or more with respectto 100% by weight of a compounded solution before gelling in order tothoroughly perform the polymerization reaction and reduce the residualmonomers, and is preferably 1.0% by weight or less in order to preventdiscoloration (yellowing) and odor due to the reaction residues of thepolymerization initiator. The amount to be added is more preferably 0.05to 0.5% by weight. Further, when polymerization is performed byultraviolet irradiation, the amount of integrated irradiation ofultraviolet light varies depending on the content of the polymerizationinitiator and the like, but, for example, the amount is preferably inthe range of 1,000 mJ/cm² to 10,000 mJ/cm², and more preferably in therange of 2,000 mJ/cm² to 10,000 mJ/cm².

As the photopolymerization initiator, those which are cleaved byultraviolet light or visible light to generate radicals are preferable,which include α-hydroxyketone, α-aminoketone, benzyl methyl ketal,bisacylphosphine oxide, metallocene and the like. More specifically, theexamples include 2-hydroxy-2-methyl-1-phenyl-propan-1-one (product name:DAROCUR 1173, manufactured by Ciba Specialty Chemicals Inc.),1-hydroxy-cyclohexyl-phenyl-ketone (product name: Irgacure 184,manufactured by Ciba Specialty Chemicals Inc.),1-[4-(2-hydroxyethoxy)-phenyl]-2-hydroxy-2-methyl-propan-1-one (productname: Irgacure 2959, manufactured by Ciba Specialty Chemicals Inc.),2-methyl-1-[(methylthio)phenyl]-2-morpholinopropan-1-one (product name:Irgacure 907, manufactured by Ciba Specialty Chemicals Inc.),2-benzyl-2-dimethylamino-1-(4-morpholinophenyl)-butan-1-one (productname: Irgacure 369, manufactured by Ciba Specialty Chemicals Inc.),2-hydroxy-1-{4-[4-(2-hydroxy-2-methyl-propionyl)-benzyl]phenyl}-2-methyl-propan-1-one(product name: Irgacure 127, manufactured by Ciba Specialty ChemicalsInc.). These can be used independently or in combination.

When the thickness of the gel sheet 1 is too small, there is difficultyin handling, and therefore an appropriate thickness is selected in viewof these aspects. The thickness is preferably in the range of 0.2 mm to2.0 mm. In particular, the thickness is preferably 0.3 mm to 1.2 mm, andmore preferably 0.5 mm to 1.0 mm.

Regarding the ratio of the thickness of the layer A 10 to that of thelayer B 20 in the gel sheet 1, when the ratio of the layer A is toolarge, there is a possibility that the adhesion of the layer B to theelectrode element may be inhibited. Conversely, when the ratio of thelayer B increases, there is a concern that the gel will run off from theside. Therefore, the ratio is appropriately determined in view ofbalancing these aspects. Specifically, the thickness ratio A/B of thelayer A 10 to the layer B 20 is preferably 0.25 to 3.5.

Moreover, in the gel sheet 1, an intermediate base material may beembedded along the in-plane direction depending on one's needs, in orderto reinforce the gel sheet and to improve the shape retaining propertywhen cutting and the like. The intermediate base material can beembedded so as to be placed between the layer A 10 and the layer B 20.Alternatively, the intermediate base material may be embedded in eitherthe layer A 10 or the layer B 20, and not between the layers.

As a specific aspect, the intermediate base material embedded in the gelsheet 1 can be constituted of a nonwoven fabric or a woven fabric. Asthe material of the nonwoven fabric and the woven fabric, natural fibersof cellulose, silk, hemp and the like, synthetic fibers of polyester,nylon, rayon, polyethylene, polypropylene, polyurethane and the like, ormixed fibers thereof can be used. A binder may be used depending onone's needs, and further, the material may be colored depending on one'sneeds.

The method for producing the nonwoven fabric is not particularlylimited, but examples thereof include a dry method, wet method, spunbondmethod, melt blown method, airlaid method, chemical bond method, thermalbond method, needle punching method, and a water-flow interlacingmethod. To control the position of the intermediate base material, it ispreferable to adopt a production method in accordance with weight perunit area and material to have an even weight per unit area. As for thewoven fabric as well, the production method is not particularly limited,and can be appropriately selected from, for example, plain weaving,tricot, raschel and the like.

Moreover, the weight per unit area of the woven or nonwoven fabric isnot particularly limited as long as it is a weight per unit area capableof obtaining predetermined physical properties as an intermediate basematerial, but is preferably 10 to 40 g/m², and more preferably 10 to 28g/m². When the weight per unit area of the woven or nonwoven fabric istoo small, the gel sheet cannot be reinforced or the like, andunevenness of the weight per unit area becomes large, so that thepermeability of liquid at the time of producing the gel sheet variesdepending on positions, and there would be a possibility that theposition of the intermediate base material fluctuate. Moreover, when theweight per unit area is too large, the intermediate base materialbecomes hard, and there would be a possibility that the followingproperty to the skin and the like of the gel sheet 1 may be impaired andthat the penetration property may be adversely affected. Therefore, theweight per unit area is appropriately determined in view of balancingthese aspects.

Moreover,a semipermeable membrane may be used as a material for theintermediate base material. This semipermeable membrane is constitutedof cellophane, cellulose acetate and the like, and it is difficult topass water and a moisturizing agent as compared to a woven fabric or anonwoven fabric, so that the compositions of each of the layer A and thelayer B at the time of production can be retained for a longer period oftime.

When the thickness of the intermediate base material is too large, thepermeability of liquid degenerates and there would be cases where thepenetration property is adversely affected. Conversely, when thethickness is too small, there would be a possibility that reinforcementand the like of the gel sheet cannot be achieved, or that the positionof the intermediate base material fluctuates, similarly to the casewhere the weight per unit area is too small. Therefore, the thickness ofthe intermediate base material is appropriately determined in view ofthese aspects. The thickness of the intermediate base material ispreferably in the range of 0.02 mm to 2.0 mm. Moreover, the thickness ismore preferably 0.02 mm to 0.5 mm, and particularly preferably 0.03 to0.3 mm.

As a method for producing the gel sheet 1, detailed conditions differdepending on the compositions of the layer A 10 and the layer B 20, thematerial and thickness of the intermediate base material and the like,and it is not particularly limited. For example, in the case ofembedding the intermediate base material, it is possible toappropriately adopt a method of holding the intermediate base materialin the air in a state in which tension of a certain magnitude or more isapplied to the intermediate base material, pouring a monomer compoundedsolution onto the upper surface and the lower surface of theintermediate base material, and thus forming a sheet by polymerizationby light irradiation or the like, a method of preparing gel materials ofa sheet-like layer A and layer B both having a smooth surface,respectively, placing the intermediate base material held in a state inwhich tension of a certain magnitude or more is applied thereto betweenthe gel materials, followed by conjugation, a method of preparing asheet-like A layer having a smooth surface, mounting the intermediatebase material onto the layer A in a state in which tension of a certainmagnitude or more is applied thereto depending on one's needs, pouring amonomer compounded solution of a layer B onto the intermediate basematerial, and further conducting polymerization by light irradiation orthe like, and the like. Moreover, it is also possible to provide a gelsheet in a roll state, and sequentially conduct the production processmentioned above.

Depending on one's needs, it is possible to provide a base film on oneside of the gel sheet 1, and to provide a top film on the side oppositeto the side provided with the base film.

As the base film, for example, a resin film made of resin such aspolyester, polyolefin, polystyrene, polyurethane and the like; paper;paper laminated with the resin film; and the like can be used.

It is preferable that the surface of the base film contacting the gelsheet 1 be subjected to release treatment. Examples of the method ofrelease treatment include silicone coating and the like, and inparticular, baking-type silicone coating which involves crosslinking andhardening reactions by heat or ultraviolet light is preferable. As afilm to be subjected to release treatment, a biaxially stretched PET(polyethylene terephthalate) film, an OPP (stretched polypropylene) filmand the like are particularly preferable.

Regarding the top film, it is basically possible to use the samematerial as that for the base film, but in the case where, in a statewhere the top film is provided, polymerization is conducted byultraviolet irradiation thereon, it is preferable to select a film of amaterial that does not block light so as not to preventphotopolymerization.

The gel sheet as described above can be used by attaching the surface ofthe layer A 10 to the skin as a bioelectrode in electroencephalographyand the like. In this case, the side of the layer B 20 is made to anelectrode element by removing the top film, and closely attaching a bodyhaving an electrically conductive layer formed by print-coating anelectrically conductive ink containing metal such as Ag, Ag/AgCl or thelike and carbon or the like on a resin film, or a body having anelectrically conductive layer formed by laminating an electricallyconductive film where metal foil (aluminum, stainless steel, Ag or thelike) or carbon or the like is kneaded on the resin film. Since thestorage elastic modulus of the layer B 20 is 2,000 to 10,000 Pa, thelayer B 20 is softer than the Layer A 10, and therefore the followingproperty to the electrode element is good and air bubbles do not developbetween the layer B 20 and the electrode element.

EXAMPLES

Hereinafter, the present invention shall be described in more detailbased on Examples and Comparative Examples, but the present invention isnot limited to these Examples.

Example 1 Composition of Layer A

With respect to the total amount of a layer A, 24% by weight ofacrylamide as a monofunctional monomer, 0.036% by weight ofN,N′-methylenebisacrylamide, 5% by weight of sodium chloride, and 40.9%by weight of glycerin as crosslinking monomers were weighed, and to thismixture solution were added 3% by weight of polyvinyl alcohol (havingdegree of saponification of 88%) and 0.1% by weight of2-hydroxy-2-methyl-1-phenyl-propan-1-one (trade name: Irgacure IR1173)as a photopolymerization initiator, and further 1% by weight each ofcitric acid 3Na and citric acid as pH buffer agents, and finallyion-exchange water was added to make the total 100% by weight, and amonomer compounded solution of the layer A was obtained.

Composition of Layer B

With respect to the total amount of a layer B, 20% by weight ofacrylamide as a monofunctional monomer, and 0.032% by weight ofN,N′-methylenebisacrylamide, 5% by weight of sodium chloride, and 44.9%by weight of glycerin as crosslinking monomers were weighed, and to thismixture solution were added 0.1% by weight of2-hydroxy-2-methyl-1-phenyl-propan-1-one (trade name: Irgacure IR1173)as a photopolymerization initiator, and further 1% by weight each ofcitric acid 3Na and citric acid as pH buffer agents, and finallyion-exchange water was added to make the total 100% by weight, and amonomer compounded solution of the layer B was obtained.

Production of Gel Sheet

The resulting monomer compounded solution of the layer B was drippedonto a silicone-coated PET film and by allowing to pass through acertain clearance, the liquid was uniformly spread out and was fixed soas to have a thickness of 0.6 mm. The liquid was irradiated withultraviolet light having an energy amount of 500 mJ/cm² using a metalhalide lamp to obtain the layer B having a thickness of 0.6 mm. Themonomer compounded solution of the layer A was dropped on the obtainedlayer B, and a similarly silicone-coated PET film was covered fromabove, and the liquid was uniformly spread out and was fixed so as tohave a thickness of 0.3 mm. The liquid was irradiated with infraredlight having an energy amount of 3,000 mJ/cm² using a metal halide lampto obtain a gel sheet having a thickness of 0.9 mm. The thickness of thegel sheet was measured by peeling off the PET film attached on both thesides using a micrometer.

Example 2

A gel sheet was prepared in the same manner as that in Example 1mentioned above, except that the thicknesses of the layer A and that ofthe layer B were 0.6 mm and 0.3 mm, respectively.

Example 3

A gel sheet was prepared in the same manner as that in Example 2mentioned above, except that the content of polyvinyl alcohol in thelayer A was 2% by weight and the content of glycerin was 41.9% byweight.

Example 4

In Example 1 mentioned above, the monomer compounded solution of thelayer B was changed as follows.

That is, with respect to the total amount of the layer B, 18% by weightof acrylamide as a monofunctional monomer, and 0.030% by weight ofN,N′-methylenebisacrylamide, 2% by weight of sodium chloride, and 39.9%by weight of glycerin as crosslinking monomers were weighed, and to thismixture solution were added 0.1% by weight of2-hydroxy-2-methyl-1-phenyl-propan-1-one (trade name: Irgacure IR1173)as a photopolymerization initiator, and further 1% by weight each ofcitric acid 3Na and citric acid as pH buffer agents, and finally 21.8%by weight (10.9% by weight of solid content, and 10.9% by weight ofwater) of an emulsion of an acrylate ester-based copolymer (50% byweight of solid content, trade name “Polysol PSA SE-1730”, manufacturedby Showa Highpolymer Co., Ltd.) as a water-insoluble polymer and 0.2% byweight of polyvinyl alcohol, with a degree of saponification of 65%,which is an amphiphilic polymer as a dispersion stabilizer. The mixturesolution was stirred for several minutes until the solution washomogeneous, and a milky white compounded solution was obtained. To themixture solution was added 16% by weight of ion-exchange water to makethe total 100% by weight, and a monomer compounded solution of the layerB was obtained.

A gel sheet was prepared in the same manner as that in Example 1, exceptthat the monomer compounded solution mentioned above was used as thelayer B.

Example 5

A gel sheet was prepared in the same manner as that in Example 4mentioned above, except that the thicknesses of the layer A and that ofthe layer B were 0.6 mm and 0.3 mm, respectively.

Example 6

A gel sheet was prepared in the same manner as that in Example 5mentioned above, except that a nylon mesh serving as an intermediatebase material was embedded between the layer A and the layer B in theproduction process of the gel sheet.

Example 7

A gel sheet was prepared in the same manner as that in Example 2mentioned above, except that a nylon mesh serving as an intermediatebase material was embedded between the layer A and the layer B in theproduction process of the gel sheet.

Example 8

A gel sheet was prepared in the same manner as that in Example 7mentioned above, except that in the monomer compounded solution of thelayer B of Example 7 mentioned above, 5% by weight of polyacrylic acidNa (an aqueous solution of 20% by weight of solid content, trade name“JURYMER AC-20H”, manufactured by Toagosei Co., Ltd.) was additionallydissolved as a tackifier, the content of acrylamide as a monofunctionalmonomer was 19% by weight, and the content of water was 24% by weight.

Example 9

A gel sheet was prepared in the same manner as in Example 8 mentionedabove, except that the compositions of the monomer compounded solutionsof the layer A and the layer B in Example 8 mentioned above were changedas shown in Table 2 and that the intermediate base material was notembedded.

Example 10

A gel sheet was prepared in the same manner as that in Example 8mentioned above, except that the compositions of the monomer compoundedsolutions of the layer A and the layer B were changed as shown in Table2.

Comparative Example 1

A gel sheet was constituted of a layer B simple substance (having athickness of 0.9 mm) not containing polyvinyl alcohol in Example 1,without forming a laminate structure.

Comparative Example 2

A gel sheet was prepared in the same manner as that in Example 2mentioned above, except that, in the layer A, polyvinyl alcohol was notcontained, the content of acrylamide as a monofunctional monomer wasreduced to 20% by weight, the content of N,N′-methylenebisacrylamide asa crosslinking monomer was increased to 0.5% by weight, the content ofwater was 28% by weight, and the content of glycerin was 44.4% byweight.

Comparative Example 3

A gel sheet was prepared in the same manner as that in Example 2mentioned above, except that an electrically conductive rubber having athickness of 2 mm (a semiconductive gel (having an Asker C hardness of15) made of urethane resin, and manufactured by Exseal Co., Ltd.) wasused as the layer A.

Comparative Example 4

A gel sheet was constituted of a layer A simple substance (having athickness of 0.9 mm) in Example 1, without forming a laminate structure.

The compositions of the gel sheets obtained in Examples 1 to 10 andComparative Examples 1 to 4 are collectively shown in Tables 1 and 2.

TABLE 1 Example 1 2 3 4 Layer A Water Ion-exchange water 25 25 25 25(Skin side) Water-soluble Polyvinyl alcohol (Degree of saponification of88%) 3 3 2 3 synthetic polymer Polyhydric alcohol Glycerin 40.9 40.941.9 40.9 (Moisturizing agent) Polymer matrix Monofunctional monomerAcrylamide 24 24 24 24 Crosslinkable monomer N,N′-methylenebisacrylamide0.036 0.036 0.036 0.036 Electrolyte salt Sodium chloride 5 5 5 5 pHbuffer agent Citric acid 1 1 1 1 Citric acid salt 1 1 1 1 Other additivePhotopolymerization initiator IR1173 0.1 0.1 0.1 0.1 Thickness of layerA (mm) 0.3 0.6 0.6 0.3 Layer Presence or absence of intermediate basematerial Absent Absent Absent Absent between layer A and layer B Layer BWater Ion-exchange water 28 28 28 16 (Element Water-soluble Polyvinylalcohol (Degree of saponification of 88%) — — — — side) syntheticpolymer Polyhydric alcohol Glycerin 44.9 44.9 44.9 39.9 (Moisturizingagent) Polymer matrix Monofunctional monomer Acrylamide 20 20 20 18Crosslinkable monomer N,N′-methylenebisacrylamide 0.032 0.032 0.0320.030 Electrolyte salt Sodium chloride 5 5 5 2 pH bufter agent Citricacid 1 1 1 1 Citric acid salt 1 1 1 1 Other additive Photopolymerizationinitiator IR1173 0.1 0.1 0.1 0.1 Tackifier (Polyacrylic acid Na) — — — —Tackifier (Acrylic acid ester copolymer) + Polyvinyl — — — 22 alcohol(Degree of saponification of 65%) Thickness of layer B (mm) 0.6 0.3 0.30.6 Example 5 6 7 Layer A Water Ion-exchange water 25 25 25 (Skin side)Water-soluble Polyvinyl alcohol (Degree of saponification of 88%) 3 3 3synthetic polymer Polyhydric alcohol Glycerin 40.9 40.9 40.9(Moisturizing agent) Polymer matrix Monofunctional monomer Acrylamide 2424 24 Crosslinkable monomer N,N′-methylenebisacrylamide 0.036 0.0360.036 Electrolyte salt Sodium chloride 5 5 5 pH buffer agent Citric acid1 1 1 Citric acid salt 1 1 1 Other additive Photopolymerizationinitiator IR1173 0.1 0.1 0.1 Thickness of layer A (mm) 0.6 0.6 0.6 LayerPresence or absence of intermediate base material Absent Present Presentbetween layer A and layer B Layer B Water Ion-exchange water 16 16 28(Element Water-soluble Polyvinyl alcohol (Degree of saponification of88%) — — — side) synthetic polymer Polyhydric alcohol Glycerin 39.9 39.944.9 (Moisturizing agent) Polymer matrix Monofunctional monomerAcrylamide 18 18 20 Crosslinkable monomer N,N′-methylenebisacrylamide0.030 0.030 0.032 Electrolyte salt Sodium chloride 2 2 5 pH bufter agentCitric acid 1 1 1 Citric acid salt 1 1 1 Other additivePhotopolymerization initiator IR1173 0.1 0.1 0.1 Tackifier (Polyacrylicacid Na) — — — Tackifier (Acrylic acid ester copolymer) + Polyvinyl 2222 — alcohol (Degree of saponification of 65%) Thickness of layer B (mm)0.3 0.3 0.3

TABLE 2 Example Comparative Example 8 9 10 1 Layer A Water Ion-exchangewater 25 21 21 28 (Skin side) Water-solable Polyvinyl alcohol (Degree ofsaponification of 88%) 3 3 3 — synthetic polymer Polyhydric alcoholGlycerin 40.9 44.9 44.9 44.9 (Moisturizing agent) Polymer matrixMonofunctional monomer Acrylamide 24 24 24 20 Crosslinkable monomerN,N′-methylenebisacrylamide 0.036 0.036 0 036 0.032 Electrolyte saltSodium chloride 5 5 5 5 pH buffer agent Citric acid 1 1 1 1 Citric acidsalt 1 1 1 1 Other additive Photopolymerization initiator IR1173 0.1 0.10.1 0.1 Thickness of layer A (mm) 0.6 0.6 0.6 0.9 Layer between Presenceor absence of intermediate base material Present Absent Present Absentlayer A and layer B Layer B Water Ion-exchange water 24 22 22 (ElementWater-solable Polyvinyl alcohol (Degree of saponification of 88%) — — —side) synthetic polymer Polyhydric alcohol Glycerin 44.9 50.4 50.4(Moisturizing agent) Polymer matrix Monofunctional monomer Acrylamide 1920 20 Crosslinkable monomer N,N′-methylenebisacrylamide 0.032 0.0360.036 Electrolyte salt Sodium chloride 5 2 2 pH buffer agent Citric acid1 1 1 Citric acid salt 1 1 1 Other additive Photopolymerizationinitiator IR1173 0.1 0.1 0.1 Tackifier (Polyacrylic acid Na) 5 3.5 3.5Tackifier (Acrylic acid ester copolymer) + Polyvinyl — — — alcohol(Degree of saponification of 65%) Thickness of layer B (mm) 0.3 0.3 0.3Comparative Example 2 3 4 Layer A Water Ion-exchange water 28 Elec- 25(Skin side) Water-solable Polyvinyl alcohol (Degree of saponification of88%) — trically 3 synthetic polymer conductive Polyhydric alcoholGlycerin 44.4 rubber 40.9 (Moisturizing agent) Polymer matrixMonofunctional monomer Acrylamide 20 24 Crosslinkable monomerN,N′-methylenebisacrylamide 0.5 0.036 Electrolyte salt Sodium chloride 55 pH buffer agent Citric acid 1 1 Citric acid salt 1 1 Other additivePhotopolymerization initiator IR1173 0.1 0.1 Thickness of layer A (mm)0.6 2 0.9 Layer between Presence or absence of intermediate basematerial Absent Absent Absent layer A and layer B Layer B WaterIon-exchange water 28 28 (Element Water-solable Polyvinyl alcohol(Degree of saponification of 88%) — — side) synthetic polymer Polyhydricalcohol Glycerin 44.9 44.9 (Moisturizing agent) Polymer matrixMonofunctional monomer Acrylamide 20 20 Crosslinkable monomerN,N′-methylenebisacrylamide 0.032 0.032 Electrolyte salt Sodium chloride5 5 pH buffer agent Citric acid 1 1 Citric acid salt 1 1 Other additivePhotopolymerization initiator IR1173 0.1 0.1 Tackifier (Polyacrylic acidNa) Tackifier (Acrylic acid ester copolymer) + Polyvinyl alcohol (Degreeof saponification of 65%) Thickness of layer B (mm) 0.3 0.3

(Evaluation of Gel Sheets)

Various tests were conducted on the gel sheets obtained in Examples 1 to10 and Comparative Examples 1 to 4.

Measurements of Adhesive Force of Gel Sheets

In accordance with JIS Z0237: 2009, test pieces were prepared by cullingeach gel sheet into 120 mm×20 mm, attaching a Bakelite plate on a gelsurface (the layer A or the layer B) that appeared after peeling off thePET film, and rolling a pressure roller of 2 kg back and forth once topressure bond the Bakelite plate. A rheometer (CR-500 DX, manufacturedby Sun Scientific Co., Ltd.) was used for measurements, under themeasuring conditions of angle of 90° and speed of 300 mm/min. Stressvalues (N/20 mm) at points from the starting point of measurement topredetermined peeling points (30, 40, 50, 60, and 70 mm) were measured,and the average value of 3 tests (total of 15 points) was taken as theadhesive force. The measurements were carried out in a measuringenvironment of a temperature of 23±5° C. and humidity of 55%±10%. Theresults of the measurements are shown in Tables 3 and 4.

Measurements of Adhesive Force of Gel Sheets (Evaluation of Feeling whenUsed)

The gel sheet was cut into 40 mm squares and the surface of the layer Awas pressed against the scalp for 1 minute. Thereafter, the gel sheetwas peeled off by hand to evaluate the degree of clinging of the hair.Signs of G (Good): when there is no pain, F (Fair): when there is nopain, but when gel adheres to the hair or when the hair is pulled, and P(Poor): when the hair falls out are used. The measurements were carriedout in a measuring environment of a temperature of 23±5° C. and humidityof 55%±10%. The results of the measurements are shown in Tables 3 and 4.

Viscoelastic Properties of Gel Sheets

The storage elastic modulus, loss elastic modulus, and loss tangent (tanθ) of each of the layer A and the layer B of the gel sheets weremeasured. Specifically, viscoelasticity of a strain amount of 1% wasmeasured at a temperature of 23° C. and a frequency of 10 Hz using aviscoelasticity measuring device (MR-102, manufactured by Anton PaarGmbH). A gel piece of 25ϕ was attached to a parallel plate of 25ϕ madeof SUS, and a jig was pressed on the gel piece until the load point of 1N, then the storage elastic modulus and the loss elastic modulus at 10Hz were measured and the loss tangent (tan θ) was calculated from thevalues. The results of the measurements are shown in Tables 3 and 4. Thehigher the storage elastic modulus is, the more it shows elasticity, andtherefore it can be used as an index of hardness.

Measurements of Hardness of Gel Sheets

Young's modulus of the gel sheet was measured by pressing a 12ϕ columnmade of SUS under the conditions of a temperature of 23±5° C. humidityof 55±10% and a test speed of 0.1 mm/min using a Tensilon UniversalTesting Machine (RTE-1210 manufactured by Orientec Co., Ltd.). An S-Scurve up to 2.98 N was measured under the conditions mentioned above.The test piece of the gel sheet was 25 mm×40 mm, and a sufficient areawas secured for the 12ϕ column.

The Young's modulus was calculated from the S-S curve from a stress ofabout 2.0 N to about 2.5 N using the following mathematical formula.

Young's modulus=difference in stress value between twopoints/cross-sectional area×(thickness of gel sheet−time when 2.0N×displacement per time)/strain amount (deformation amount from 2.0 N to2.5 N)

Measurements of Shore Hardness of Gel Sheets

Asker C hardness of the surfaces of the layer A and the layer B of thegel sheet was measured in accordance with JIS K 7312. The results of themeasurements are shown in Tables 3 and 4. As shown in Tables 3 and 4,the Asker C hardness of the surface of the layer A was 9 to 42, and thesurface was dented by a convex portion pressed at the time ofmeasurement, but it was restored. Moreover, the Asker C hardness of thesurface of the layer B was 0, and the surface was scratched by a pressedconvex portion.

Measurements of Stress Relaxation Force of Gel Sheets

A 12ϕ column made of SUS was pressed under the conditions of atemperature of 23±5° C., humidity of 55±10% and a test speed of 0.1mm/min using a Tensilon Universal Testing Machine (RTE-1210 manufacturedby Orientec Co., Ltd.). After applying a displacement until reaching 40N, the displacement was fixed at 40 N and was kept for 40 sec. Thestress relaxation value at the time was measured. The obtained value wasdivided by 40 N to show in terms of ratio how much stress valuedecreased with respect to the applied stress.

Electric Performance Evaluation of Gel Sheets

First, the gel sheets were separated into the layer As and the layer Bs.Then, the layer As and the layer Bs were each cut into 20 mm squares,and the resulting A layers or B layers were overlapped with each otherto prepare test pieces. The test pieces were placed between SUS plates,and the alternating current impedance (Ω) of each of the layer A and thelayer B was measured under the conditions of a storage oscilloscope (aninput voltage of 10 V, frequency of 10 Hz, resistance of 1 MΩ). Theresults of the measurements are shown in Tables 3 and 4.

TABLE 3 Example 1 2 3 4 5 6 7 Adhesive force Layer A 1.1 0.8 1.4 1.1 1.11.1 1.1 (N/20 mm) Layer B 5.4 4.7 4.3 11 10.6 10.8 4.2 Adhesive forceLayer A G G F G G G G (Feeling when used) Storage elastic Layer A 1301222899 15260 14562 23451 22961 23113 modulus (Pa) Layer B 7005 4139 40549188 8582 8425 4352 AC impedance (Ω) Layer A 450 386 381 452 347 344 342Layer B 11.2 15.0 15.4 340 287 255 13.2 Young's modulus Layer A 0.460.33 0.28 0.46 0.33 0.33 0.33 (MPa) Layer B 0.25 0.25 0.25 0.26 0.260.26 0.25 Asker C hardness Layer A 9 Without 9 Without 9 Without 9Without 9 Without 9 Without 9 Without scratch scratch scratch scratchscratch scratch scratch Layer B 0 With 0 With 0 With 0 With 0 With 0With 0 With scratch scratch scratch scratch scratch scratch scratchStress relaxation Layer A 37 37 45 37 37 37 37 (%) Layer B 50 50 50 4747 47 50

TABLE 4 Example Comparative Example 8 9 10 1 2 3 4 Adhesive force LaverA 1.1 1.2 1.2 6.2 0.3 0.2 1.4 (N/20 mm) Layer B 7.2 3.8 3.6 4.2 3.8Adhesive force Layer A G G G P *1 G G (Feeling when used) Storageelastic Layer A 23094 23511 23651 9850 *1 56666 31520 modulus (Pa) LayerB 3874 8074 8182 3878 4112 AC impedance (Ω) Layer A 355 410 408 32 240*2 520 Layer B 12.2 352 381 150 150 Young's modulus Layer A 0.33 0.330.33 0.25 1.52 0.43 0.5 (MPa) Layer B 0.25 0.25 0.25 0.25 0.25 Asker Chardness Layer A 9 Without 9 Without 9 Without 0 With 42 With 15 Without9 Without scratch scratch scratch scratch scratch scratch scratch LayerB 0 With 0 With 0 With 0 With 0 With scratch scratch scratch scratchscratch Stress relaxation Layer A 37 37 37 50 *3 22 37 (%) Layer B 50 4848 50 50 *1 Handling of gel was bad and damaged. *2 Over ranged *3Unmeasurable due to damage.

As shown in Tables 3 and 4, according to the present invention, gelsheets showing different behaviors in respective layers were obtained,and adequate physical properties were exhibited on the skin side and theelectrode element side, respectively.

The gel sheets obtained in the Examples and the Comparative Exampleswere laminated on electrode elements obtained by laminating electricallyconductive films where carbon has been kneaded on resin films. A gelsheet obtained in Comparative Example 4 was too hard, exhibited lowadhesive force, and showed no following property to the electrodeelement and adhesion was poor.

REFERENCE SIGNS LIST

-   1 A gel sheet-   10 Layer A-   20 Layer B

All publications, patents and patent applications cited in thisspecification shall be incorporated herein by reference in theirentirety.

1. A gel sheet having a laminated structure of a layer A and a layer B, wherein a storage elastic modulus of the layer A at 23° C. and 10 Hz is 12,000 to 40,000 Pa, and a storage elastic modulus of the layer B at 23° C. and 10 Hz is 2,000 to 10,000 Pa.
 2. The gel sheet according to claim 1, wherein an adhesive force of the layer A to a Bakelite plate is 0.1 to 2.0 N/20 mm, and an adhesive force of the layer B to the Bakelite plate is 2.5 to 15 N/20 mm.
 3. The gel sheet according to claim 1, having a thickness of 0.2 mm to 2.0 mm.
 4. The gel sheet according to claim 1, wherein a ratio of a thickness of the layer A to a thickness of the layer B (A/B) is 0.25 to 3.5.
 5. The gel sheet according to claim 1, wherein the layer A and the layer B are hydrogels comprising a polymer matrix, water and polyhydric alcohol, and the polymer matrix is a copolymer of one or more monofunctional monomers selected from a (meth)acrylamide-based monomer and a (meth)acrylate ester, and a crosslinkable monomer.
 6. The gel sheet according to claim 5, wherein the layer A further comprises polyvinyl alcohol.
 7. The gel sheet according to claim 1, wherein an alternating current impedance of the layer A and that of the layer B at 10 Hz are both 1,000 Ω or less.
 8. The gel sheet according to claim 1, being an adhesive gel sheet for skin to be used by attaching a surface of the layer A to the skin. 